Modulatory actions of noradrenaline (NA) and serotonin (5-HT) on spinal interneuronal circuits will be investigated in the cat. NA and 5-HT agents assist in recovery after spinal injuries but little is known about the mechanisms of their actions at pre-motoneuronal levels. Previous studies only demonstrated which synaptic actions of peripheral afferents on spinal inteneurons are facilitated or depressed. The 1st series will define subtypes of receptors involved in facilitating and depressing interneuronal responses. In acute experiments on deeply anaesthetised cats selective NA and 5-HT receptor agonists and antagonists will be applied locally (ionophoretically). The ability of these agents to modify extracellularly recorded responses of premotor inteneurons co-ordinating activity of flexors and extensors of a limb will then be investigated. The aim of the 2nd series will be to investigate interactions between effects of simultaneously applied NA and 5-HT. These will be analyzed on the same functional types of inteneurons as in the first sties. A comparison will be made between effects of 5-HT and NA, or their most potent selective receptor agonists, applied both alone and together. The aim of the 3rd series will be to investigate the distribution of at least some of the receptors found to mediate modulatory actions of NA and 5- HT on presynaptic afferents and on the soma and dendrites of the interneurons in a morphological study by using antibodies against these receptors. The 4th series will combine the analysis of effects of monoamines on commissural interneurons which coordinate activity of the two limbs, with the identification of excitatory and inhibitory interneurons, a morphological analysis of their axonal projections and the distribution of monoaminergic contacts on them.